Electric shutter operating circuits

ABSTRACT

In an electronic shutter operating circuit for use in a photographic camera a differential amplifier comprising a pair of transistors is provided for comparing the photosensitivity of the film used and the brightness of the object to display over exposure and under exposure conditions. A voltage is positively fed back from the output of the differential amplifier to the input thereof to cause it to oscillate for displaying the correct exposure condition. There is provided a transfer switch to cause the differential amplifier to respond to the selected shutter time for closing the shutter when the shutter time expires.

BACKGROUND OF THE INVENTION

This invention relates to an electric shutter operating circuit for usein photographic cameras.

The electric shutter operating circuit for use in photographic camerasis constructed to connect the amount of the light received from anobject into an electric signal, to determine the photographingconditions commensurate with the brightness of the object in accordancewith the electrical signal, to indicate to the user whether the irisopening and the shutter time are correct or not and to control theoperation of the shutter.

In the past, various types of electric shutter operating circuits havebeen proposed, but prior art circuits were constructed to indicate thecorrect value or incorrect value of the exposure for a particularbrightness of the object by merely representing over exposure and underexposure or correct exposure to the user of the camera. Accordingly, inthe former case, absence of the display of the over or under exposuremeans the correct exposure. However, the correct exposure is also notdisplayed in a case of the fault of electric power source or electriccircuit thus resulting in the failure of photographing or giving afeeling of uncertainty to the user. In the latter case, where theexposure is incorrect, the user cannot judge whether the exposure isover or under.

Furthermore, in the prior art electric shutter operating circuit, as theshutter control circuit and the exposure display circuit were providedindependently, there was a defect that the entire circuit wascomplicated and expensive.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedelectric shutter operating circuit of relatively simple circuitconstruction for use in a photographic camera that can display not onlythe over exposure and under exposure conditions but also the correctexposure condition.

Another object of this invention is to provide an improved electricshutter operating circuit which can not only display the over exposure,under exposure and correct exposure conditions, but also control theshutter time.

According to this invention, these objects can be accomplished byproviding an electric shutter operating circuit for use in aphotographic camera, comprising means responsive to the brightness of anobject to be photographed, a first transistor normally connected torespond to the output of the brightness responsive means, a resistor setin accordance with the photosensitivity of the photographic film used, asecond transistor connected to respond to the output voltage of theresistor, means to connect the first and second transistors to act as adifferential amplifier, means responsive to the output of thedifferential amplifier which is produced when the differential amplifieris unbalanced for operating display means to display an over exposure oran under exposure condition, means responsive to the balanced conditionof the differential amplifier for providing a positive feedback to thedifferential amplifier thereby causing the same to oscillate, and meansresponsive to the oscillation of the differential amplifier fordisplaying a correct exposure condition.

According to another feature of this invention, the electric shutteroperating circuit just described further comprises a variable resistorwhich is set in accordance with a predetermined shutter time, acapacitor connected to be charged through the variable resistor, aswitch normally short circuiting the capacitor and arranged to be openedwhen the shutter of the camera is opened, an electromagnet associatedwith the shutter and connected on the output side of the differentialamplifier, and a transfer switch operated concurrently with saidcapacitor switch for connecting the first transistor to be responsive tothe terminal voltage of the capacitor so that as the capacitor ischarged up the differential amplifier operates the electromagnet toclose the shutter.

BRIEF DESCRIPTION OF THE DRAWING

A single FIGURE of the accompanying drawing shows a connection diagramof one embodiment of the novel electric shutter operating circuitembodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the accompanying drawing, the electric shutteroperating circuit shown therein comprises a photoconductive elementR_(x) which varies its resistance in accordance with the brightness ofthe object, not shown, and is connected in series to a variable resistorRV₁, a switch SW₁ which is arranged to be opened when a shutter, notshown, of a photographic camera is operated, and a capacitor C₁connected in parallel with the switch SW₁, the capacitor C₁ beingconnected to be charged through a variable resistor RV₂ interlocked withthe variable resistor RV₁ when the switch SW₁ is opened. There is alsoprovided a transfer switch SW₂ for switching the connection between thecharging voltage of the capacitor C₁ and the voltage created by thephotoconductive element R_(x). Although not shown in the drawing thetransfer switch SW₂ is interlocked with a release button, not shown, ofthe camera such that the movable contact of the transfer switch isnormally thrown to contact, a, connected to the photoconductive elementR_(x), but thrown to contact, b, connected to capacitor C₁ in responseto the opening operation of the shutter. The voltage appearing at themovable contact of the transfer switch SW₂ is impressed upon the baseelectrode of a transistor Q₁ which constitutes a differential amplifiertogether with the other transistor Q₂. The base electrode of transistorQ₂ is connected to the sliding arm of a variable resistor RV₃ which isvaried in accordance with the photosensitivity (ASA) of the photographicfilm used. The variable resistor RV₃ is connected, in series to a seriesconnection of diodes D₂ and D₃ adapted to compensate for the temperaturecharacteristics of transistor Q₂, across a source conductor L₁ and aconductor L₂ maintained at a reference or ground potential. A transistorQ₃ acting as a source of constant current is connected to the emitterelectrodes of both transistors Q₁ and Q₂ and the base electrode of thetransistor Q.sub. 3 is connected to the juncture between diodes D₂ andD₃ so as to be impressed with the forward voltage of the diode D₃thereby maintaining a constant current. The other or lower terminals ofswitch SW₁, capacitor C₁ and photoconductive element R_(x) are connectedto the conductor L₂ of the reference potential through a diode D₁ whichis connected to the source conductor L₁ via a resistor R₁ so that saidother terminals of switch SW₁, capacitor C₁ and photoconductive elementR_(x) are maintained at a higher voltage than the reference potential bythe forward voltage drop across the diode D₁. The polarity of thecollector output of transistor Q₁ is inverted by the operation ofcascade connected transistors Q₄, Q₅ and Q₆ and is then applied to oneterminal of resistor R₂ connected between the emitter electrode oftransistor Q₆ and conductor L₂ of the reference voltage.

The polarity of the signal generated across resistor R₂ is inverted by atransistor Q₇ having its emitter electrode grounded, and the polarity ofthe signal generated at the collector electrode of the transistor Q₇ isinverted by a transistor Q₈ with its emitter electrode grounded.Further, the output from the collector electrode of transistor Q₇ iscoupled to the base electrode of a transistor Q₉ through a diode D₄ forpreventing reverse current, the emitter electrode of transistor Q₉ beingalso grounded. A capacitor C₃ is connected across the base-emitterelectrodes of transistor Q₉ such that when transistor Q₇ isnonconductive, diode D₄ is disconnected and capacitor C₃ is charged fromthe source conductor L₁ through a resistor R₆ with a time constant of R₆C₃, and that as transistor Q₉ is turned on by the terminal voltage ofcapacitor C₃ a display lamp LD₁ for over exposure is lighted.

On the other hand, the collector output of transistor Q₈ is coupledthrough a diode D₅ to the base electrode of transistor Q₁₁ with itsemitter electrode grounded so that when transistor Q₈ is rendered off,the diode D₅ is disconnected and the capacitor C₅ is charged by thesource conductor L₁ via a resistor R₈. When transistor Q₁₁ is renderedon a display lamp LD₃ for the under exposure is lighted. As described indetail later, an alternating collector output of transistor Q₈ isapplied through a capacitor C₄ to the base electrode of a transistor Q₁₀with its emitter electrode grounded so that transistor Q₁₀ is renderedalternately on and off, thereby alternately lighting and extinguishing adisplay lamp LD₂ for a correct exposure. To this end, the collectoroutput of transistor Q₈ is fedback to the base electrode of transistorQ₂ through an integrating circuit comprising resistor R4 and capacitorC₂. The upper terminal of capacitor C₂ is connected to the collectorelectrode of transistor Q₇. It is constructed such that the timeconstant provided by resistor R₄ and capacitor C₂ is sufficientlysmaller than the time constants provided by resistor R₆ and capacitorC₃, and resistor R₈ and capacitor C₅ and that the resistance value ofvariable resistor RV₂ varies exponentially.

The source conductor L₁ is connected to one pole of a DC source Ethrough a source switch SW₃. The lighting and extinguishment ofrespective display lamps LD₁, LD₂ and LD₃ illuminate suitable displayletters, arrows or the like symbols of a display board 1 to display thedegree of exposure.

The electric shutter operating circuit described above operates asfollows. At first switch SW₁ is closed, and the movable contact of thetransfer switch SW₂ is thrown to contact, a, connected to thephotoconductive element R_(x) and the variable resistor RV₃ is set to apredetermined value in accordance with the photosensitivity ASA of thefilm. Under these conditions, when the release button is depressed,source switch SW₃ is closed to connect the source line L₁ to the DCsource E. As the photoconductive element R_(x) manifests a resistancevalue corresponding to the brightness of the object, the base voltage oftransistor Q₁ will be given by an equation ##EQU1## where V_(b1)represents the voltage between the base electrode of transistor Q₁ andthe anode electrode of diode D₁, r_(x) the resistance value of thephotoconductive element R_(x), r_(v1) the resistance value of thevariable resistor R_(v1), and e the voltage value of the source E. Thevoltage V_(b2) between the base electrode of transistor Q₂ and the anodeelectrode of diode D₃ is determined by the variable resistor RV₃ andthere is a relation V_(b1) + V_(off) = V_(b2) between the voltagesV_(b1) and V_(b2), this relation maintaining transistors Q₁ and Q₂ in anequilibrium condition, where V_(off) represents the offset voltage ofthe transistors.

Where the brightness of the object is large to satisfy a relation V_(b1)< V_(b2) - V_(off), then transistors Q₁, Q₄, Q₅, Q₆ and Q₇ are allrendered off, whereas transistor Q₈ is on. In consequence transistor Q₉is rendered on and transistors Q₁₀ and Q₁₁ off. The on-state oftransistor Q₇ lights display lamp LD₁ to illuminate an arrow 2 on thedisplay board 1 so as to indicate that the combination of the shutterspeed and the opening of the diaphragm set at that time is not suitablethus causing an over exposure. This means that the user must decreasethe opening of the iris diaphragm or increase the shutter speed.

On the other hand when the brightness of the object is low satisfying arelation V_(b1) > V_(b2) - V_(off), then transistors Q₁, Q₄, Q₅, Q₆ andQ₇ are all rendered on whereas transistor Q₈ is off. As a result,transistor Q₉ is rendered off, transistor Q₁₀ is also off, and onlytransistor Q₁₁ is rendered on. The on-state of transistor Q₁₁ lightsdisplay lamp LD₃ to illuminate arrow 4 on the display board 1, therebyalarming that the combination of the presently set shutter speed and theopening of the iris diaphragm causes an under exposure.

In response to the alarm and request described above, when the useradjusts the diaphragm opening 5 or shutter speed, that is variableresistor RV₁, transistors Q₁ and Q₂ will approach a balanced condition.When the balanced condition V_(b1) = V_(b2) - V_(off) is reached, theoperation of the circuit becomes quite unstable. More particularly, whentransistor Q₇ is turned on and transistor Q₈ is turned off, or whentransistor Q₇ is turned off and transistor Q₈ is turned on,notwithstanding the transient time constant provided by capacitor C₂ andresistor R₄, a voltage is positively fed back to the base electrode oftransistor Q₂ and when the amount of feedback reaches a value thatupsets the balanced state, an oscillation will result. The reason whythe differential circuit is inverted by the fedback voltage thereby toprevent oscillation under the stable condition, that is, in the case ofover exposure or under exposure, is as follows. Thus, in the case ofover exposure, the base potential of transistor Q₂ is higher than thebase potential of transistor Q₁ thereby turning off transistor Q₁,turning on transistor Q₂, turning off transistor Q₇ and turning on thetransistor Q₈ so that the upper terminal of capacitor C₂ connected tothe collector electrode of transistor Q₇ is charged to the voltage ofthe source E whereas the lower terminal is charged to the base voltageof transistor Q₂. Then resistor R₄ is connected in parallel with thevariable resistor RV₃ across the base electrode of transistor Q₂ and theground with the result that the fractional voltage provided by thevariable resistor RV₃ is decreased by the parallel connection ofresistor R₄. However, since the circuit constants are selected such thatsaid decrease in the fractional voltage will be smaller than thedifference between the base potentials of transistors Q₁ and Q₂, thedifferential circuit will not invert. On the other hand, in the case ofunder exposure, the base potential of transistor Q₂ is lower than thebase potential of transistor Q₁ so that transistor Q₁ is turned on,transistor Q₂ is turned off, transistor Q₇ is turned on and transistorQ₈ is turned off. Accordingly, the upper terminal of capacitor C₂ whichis connected to the collector electrode of transistor Q₇ is charged tothe ground potential whereas the lower terminal of C₂ has the basepotential of transistor Q₂. Then resistor R₄ is connected in parallelwith the variable resistor RV₃ between the base electrode of transistorQ₂ and the positive pole of the source E, thus increasing the basepotential of transistor Q₂. However, since the circuit constants areselected such that this increase in the base potential of transistor Q₂will be smaller than the difference between the base potentials oftransistors Q₁ and Q₂, in this case too, the differential circuit willnot be inverted.

Under the oscillation condition, transistors Q₇ and Q₈ are repeatedlyturned on and off thus applying an alternating voltage to the baseelectrodes of transistors Q₉ to Q₁₁ inclusive. Since the time constantsprovided by resistor R₆ and capacitor C₃, and by resistor R₈ andcapacitor C₅, respectively, are made to be larger than the time constantprovided by resistor R₄ and capacitor C₂, the base potentials oftransistors Q₉ and Q₁₁ are maintained near the reference potential sothat these transistors are maintained in their off-state. Since the baseelectrode of transistor Q₁₀ is impressed with the alternating voltagethrough capacitor C₄, transistor Q₁₀ is alternately rendered on and off.As a result, only the display lamp LD.sub. 2 is alternately energizedand deenergized to display the correct exposure by symbol 3 on thedisplay board 1.

As has been described hereinabove, when the movable contact of transferswitch SW₂ is thrown to contact b connected to capacitor C₁ in responseto the opening motion of the shutter while the variable resistor RV₁ orthe opening of the diaphragm 5 is adjusted to a proper value, the switchSW₁ is opened in response to the movement of the blades of shutter.Then, the voltage V_(b1) across the capacitor C₁ increases with a timeconstant determined by the values of variable resistor RV₂ and capacitorC₁ and at an instant when transistor Q₈ is turned on an electromagnet Mgis energized to release the blade whereby the shutter is closed and thephotographing operation is completed.

With the circuit construction described above a comparator circuitconstituted by the differential amplifier operates to compare themagnitudes of the voltages respectively corresponding to thephotosensitivity of the film used and the brightness of the object fordisplaying over exposure and under exposure conditions. Further, in thecase of a correct exposure, a voltage is positively fed back to thedifferential amplifier to cause it to oscillate thus displaying thecorrect exposure condition. It is also possible to control the shutterspeed by setting the shutter time in accordance with the operation forestablishing a correct exposure condition and by switching the transferswitch immediately prior to the operation of the shutter.

Although in the illustrated embodiment a photoconductive element wasused as means for detecting the brightness of the object, it will beclear that the photoconductive element may be substituted by any othercircuit network or device that varies its output current in accordancewith the amount of light received, for example a constant currentcircuit including a memory circuit.

What is claimed is:
 1. An electric exposure circuit for use in aphotographic camera having a shutter and an over and under exposuredisplay means, comprising means responsive to the brightness of anobject to be photographed for producing a first signal representing saidbrightness, a first transistor normally connected to respond to saidfirst signal of said brightness responsive means, an adjustable resistorsettable in accordance with the photosensitivity of the photographicfilm used, a voltage divider circuit including said adjustable resistorand a second transistor connected to respond to the output voltage ofsaid adjustable resistor, circuit means connecting said first and secondtransistors as a differential amplifier for comparing the relativemagnitude of said voltage and said first signal, means responsive to thecomparison output signal of said differential amplifier which isproduced when said differential amplifier is unbalanced in oppositedirections for operating said over and under exposure display means todisplay respectively an over exposure or an under exposure condition, apositive feedback circuit connected to said differential amplifier,means responsive to the balanced condition of said differentialamplifier for actuating said positive feedback circuit to cause saiddifferential amplifier to oscillate, and means responsive to theoscillation of said differential amplifier for displaying a correctexposure condition.
 2. The electric shutter exposure circuit accordingto claim 1 which further comprises a second variable resistor which isset in accordance with a predetermined shutter time, a capacitorconnected to be charged through said second variable resistor, a firstswitch connected for normally short circuiting said capacitor andarranged to be opened substantially at the time that said shutter of thecamera is opened, an electromagnet coupled for operating said shutterand connected on the output side of said differential amplifier, and atransfer swtich operated concurrently with said first switch forconnecting said first transistor to be responsive to the terminalvoltage of said capacitor, so that as said capacitor is charged up saiddifferential amplifier operates said electromagnet to close saidshutter.
 3. The electric shutter exposure circuit according to claim 1wherein said last mentioned means comprises a capacitor and a transistorconnected to respond to said comparison output signal of saiddifferential amplifier through said capacitor.
 4. The electric shutterexposure circuit according to claim 1 wherein said positive feedback isapplied through a time constant circuit having a predetermined timeconstant and said means responsive to the unbalanced condition of saiddifferential amplifier has a time constant larger than that of said timeconstant circuit.
 5. In an electric shutter exposure circuit for use ina photographic camera having a shutter with an iris diaphragm, avariable shutter time setting circuit, a light quantity informationcircuit including means for adjusting the quantity of light receivedfrom an object in accordance with the opening of said iris diaphragm ofsaid camera and said shutter time setting circuit, a comparator circuitfor comparing one of the light quantity information signals produced bysaid light quantity information circuit or a shutter time informationproduced by said shutter time setting circuit with a predeterminedreference quantity for producing a non-oscillatory over exposure signalor under exposure signal, circuit means for producing an over exposureindication and under exposure indication corresponding respectively tosaid over exposure signal and under exposure signal produced by saidcomparator circuit the improvement characterized by providing a correctexposure indication when said light quantity information circuitproduces a correct exposure signal, comprising the steps of positivelyfeeding back a voltage to said comparator circuit through a first timedelay for causing said comparator circuit to oscillate in response tosaid correct exposure signal thereby producing an alternating currentsignal, providing a time delay larger than said first time delay forsaid over exposure signal and said under exposure signal to suppressindication thereof in the presence of said alternating current signal,and displaying the presence of said alternating current signal as anindication of a correct exposure condition.